Laser printers that use a modulated laser beam to write images on a photosensitive or thermally sensitive medium such as film or paper are well known in the art. It is common practice to employ a means for deflecting the modulated laser beam into a raster pattern on said medium so that the image will be written line by line in a raster fashion. FIG. 1 shows an example of a configuration of optical components which is fairly typical of such a laser printer. Any given laser printer design may have fewer or more numerous components, but will bear a general resemblance to the configuration shown in FIG. 1. The operation of the laser printer 10 is as follows.
An optical coupler 12 contains a source of laser energy (such as the end of a fiber optic 13 through which laser light is conducted from a laser diode 15, or the laser diode itself) and a lens (not shown). The purpose of the lens in the optical coupler is to collect the laser energy emitting from the source and focus it to a beam waist 14. The laser energy which shall now be referred to as a laser beam, is propagated in a direction which is commonly referred to as the "Z axis direction". As the beam travels from the waist 14, the size of the beam becomes larger; it diverges. The laser beam next encounters cylindrical mirror 16, which is inclined, causing the axis of the reflected beam to be at an angle .alpha. with respect to the axis of the incident beam. The cylindrical shape of mirror 16 causes the beam to be focused on a print medium 19 in the scan direction A but has no effect on the beam in the cross-scan direction B. The beam next encounters cylindrical mirror 18 which is also inclined, causing the axis of the reflected beam to be at an angle .beta. with respect to the axis of the incident beam. Typically the angles .alpha. and .beta. will be equal so that the axis of the beam as it leaves mirror 18 will be parallel to the axis of the beam as it approaches mirror 16. The purpose of mirror 18 is to focus the beam on the deflecting mirror 20 in the "cross-scan" direction (orthogonal to the scanning direction). The deflecting mirror 20 is shown to be a polygon with axis of rotation C, but it could have been a single mirror mounted on the shaft of a galvanometer. The purpose of the rotation of mirror 20 about axis C is to produce the scan lines 22 on the print medium 19. After being reflected off mirror 20 the beam next encounters flat mirror 24, which is inclined so as to direct the beam toward cylindrical mirror 26. The beam continues to converge in the scan direction but now diverges in the cross-scan direction. Next, the beam encounters cylindrical mirror 26 which is inclined so as to direct the beam toward the print medium 19. The cylindrical shape of mirror 26 is designed to image the polygon (mirror 20) onto the scan line 22. This renders the system insensitive to so called "pyramid error" of the polygon. Finally, the print medium 19 is moved at a steady speed in the cross-scan direction B causing successive lines to be written at a uniform spacing on the medium.
It is a characteristic of printers of the type just described to be capable of writing extremely high resolution images having extremely fine lines and sharp detail in both the scan direction and the cross-scan direction. However, in order to achieve the very high resolution of which the printer is capable, all of the optical components must be very precisely manufactured and positioned. Even a positional error of a few microns can cause a noticeable degradation of the printer's performance. For this reason, it is of the greatest importance in the manufacture of such precision apparatus that great care be taken in assuring that critical components be very accurately and precisely mounted in place.
There are different approaches that have been taken in the factory to achieve this result. One approach is to specify very tight dimensional tolerances on all parts having dimensions which contribute to the determination of the final location of components required to be precisely and accurately mounted. Unfortunately, this approach can be expensive. Parts with very tight dimensional tolerances can cost a lot more than parts with ordinary commercially typical tolerances.
Another approach is to design position adjustment mechanisms into the apparatus which have knobs that can be used to "dial in" the exact position of the critical components. For example, a knob might be provided on the apparatus which, when turned, would cause the laser to move along the optical axis (the Z axis), allowing its precise focal position to be set. Other knobs might be provided to make adjustment in other critical directions, such as X and Y. Also, knobs might be provided to make rotational adjustment of critical components. It is easy to imagine that if a component were to be mounted so that its precise position could be set with knobs, those knobs with their associated hardware and brackets could get to be fairly bulky and complicated. Also, all this additional hardware would add considerable additional expense to the apparatus.
Another approach is to design the positioning hardware and knobs to be part of a factory fixture, then fixing the precision components in place on the apparatus after the components have been precisely aligned, for example by using an adhesive such as potting compound or epoxy. In this way, the apparatus is kept inexpensive by using parts with ordinary commercially typical tolerances while still achieving very accurate and precise positional location of the critical components. The factory fixture would, however, be fairly elaborate and expensive, since it would contain the position adjusting hardware and knobs. However, this fixture would be used over and over again to align many, many products. So its expense, when divided among the many products aligned on it, would be insignificant.
There are, however, drawbacks of using adhesive to fix the parts in position. One of them is that one must wait for the adhesive to cure before removing the hardware apparatus from the fixture. This requires that the apparatus must reside on the fixture for some additional time while curing takes place. This, of course, reduces the number of hardware units which can be manufactured by this fixture in a given span of time. To improve this situation, quick curing adhesive may be specified, such types as "5 minute curing" epoxy or "instant UV curing" adhesive. The idea with using the UV curing adhesive is to get instant curing (but weak strength) while the apparatus is on the fixture, then carefully remove it from the fixture and apply some additional adhesive which is more durable and allow overnight curing off line. For many purposes, the 5 minute curing epoxy is durable enough to provide a lasting bond. Fixing with adhesive also has some other drawbacks. One drawback is that as the adhesive cures, it shrinks somewhat. This may produce some unwanted residual stress or positional error in the location of the component(s) being assembled. Some other drawbacks of adhesive are that it must be carefully stored and used so as not to have it "go bad" on the shelf or in the pot. Adhesives have limited shelf life and for reasons just previously discussed, a very short pot life. Still another disadvantage of adhesive assembly is that after an apparatus is assembled, if any component fails, then rework is difficult if not impossible. Once the adhesive is cured, the parts cannot be disassembled and reassembled. The adhesive forms a permanent bond.
The optical performance of the laser printer is especially sensitive to the spacing and angles between the coupling 12 and the mirrors 16 and 18 (the so called pre-deflector components), so these must be precisely controlled. Unfortunately, the tolerances of the components are large enough so that even if one could position each component perfectly in its nominal designed position, the optical performance of the system would be sub-optimum. In addition to the tolerances on the components themselves, we must allow for tolerances in the mechanical hardware which will be used to mount these components. To achieve the necessary degree of accuracy in the positioning of the pre-deflector components it is customary to use a combination of fine micrometer adjustments and mounting hardware which is manufactured to very tight tolerances. The unfortunate result of this approach is that it is expensive because of both the tight tolerances and the "on board" adjustments.
There is a need therefore for an improved method of positioning the optical components in a laser printer. In particular, there is a need for achieving the very precise position of the "pre-deflector" components (optical coupler 12, mirror 16, and mirror 18).